Leflunomide, N-(4-trifluoromethylphenyl)-4-carboxamidyl-5-methylisoxazole, is an isoxazole derivative developed by Aventis Pharmaceuticals for treating Rheumatoid Arthritis that was recently introduced on the market:

Recently, leflunomide and some of its analogs were tested for several cancer cell lines as described in U.S. Pat. No. 5,977,151. This work claims that certain leflunomide analogs can be used to treat lung cancer, leukemia, Kaposi's sarcoma, ovarian cancer, sarcoma, meningioma, intestinal cancer, cancer of the lymph nodes, brain tumors, breast cancer, stomach cancer, cancer of the pancreas, cancer of the prostate, and skin cancer.
However, leflunomide is known for its toxic side effects, such as hepatoxicity and lymphoma. For example, leflunomide has caused 12 deaths by liver failure in the U.S. and 5 deaths by intestinal pneumonia in Japan. Thus, as a therapeutic agent for cancer treatment, leflunomide could be significantly improved.